Ensure Description is populated for multi-arch images

[bschwedler]

I noticed this message on GHCR:

We will need to ensure that the description annotation is set correctly when creating the multi-platform merged manifest.

We should also check whether there are other annotations that we should be pulling out in this manner and adding back into the merged manifest.

[ianpittwood]

I was curious about this and started looking into it. Unfortunately, there does not appear to be a direct way to apply "custom" (aka non-platform or OS based annotations) outside of builds with the docker CLI.

Our next best option appears to be to use an alternate library for managing our OCI indices and artifacts. This led me to stumble upon ORAS, which appears to do a lot of this stuff we've been stumbling on for multiplatform image support! It also has a Python library available. It seems like using a library/tool like this could solve multiple issues at once.